1. Field of the Invention
The present invention relates to bicycle suspension systems and more particularly to a lightweight suspension fork assembly. This invention is an improvement on bicycle suspension systems that allows for a torsionally stiff suspension system that is very lightweight. The system addresses drawbacks of conventional designs.
2. Description of the Prior Art
Various suspension systems have been proposed and developed for bicycles. Many of these systems utilize a pair of telescoping assemblies between which the front wheel is mounted. Each assembly comprises an outer tube and an inner tube which is free to move in and out of the outer tube and is cushioned by a damper of one sort or another. The outer tubes are connected at the lower ends to the bicycle axle of the front wheel and the upper ends of the inner tube are connected together in a fashion similar to the usual upper end of a bicycle fork.
As is known to those skilled in the art, these types of suspension systems use anti-friction bushings to allow free movement of the inner tube within the outer tube. These bushings have undesirable static friction called “stiction.” Because of this, the suspension systems using such bushings tend to stick and release. In addition, the two telescoping assemblies also have to be fixed together in some manner as through a “U” shaped yoke at the upper ends of the tubes to eliminate twisting. Even with this “U” shaped yoke the torsional stiffness of these types of assemblies is still limited.
In addition the use of two sets of telescoping fork tubes and a steerer tube add considerable weight to the suspension system. Most of these suspension systems utilize damping mechanisms housed in each fork tube to provide compression and rebound damping. The fork tubes are filled with oil which adds considerable weight.
A prior art example of a system which overcomes stiction is shown in Farris et al. U.S. Pat. No. 5,320,374 and subsequent applications. In this example a different form of suspension system is described using an outer tube which is adapted to be mounted in and extend through the head tube of the bicycle frame and an inner tube connected to the fork of the bicycle which telescopes within the outer tube. The inner surface of the outer tube and the outer surface of the inner tube each have a plurality (at least three) of axially arranged opposing longitudinal flat sections such as four on each tube. A plurality of hardened steel inner race shims are positioned longitudinally on the flats of the inner tube. A plurality of hardened steel outer race shims are positioned longitudinally on the flats of the outer tube. A plurality of needle bearings are disposed between the tubes in between the respective inner and outer race shims. This arrangement allows the two tubes to freely telescope in and out with respect to one another without any significant static friction and also serves to transmit the torsional steering force from the outer tube to the inner tube. This particular system is used extensively today because it can bear a combination of loads comprising very high radial loads and at the same time provide stable and tight rotational motion in steering of the front wheel through the suspension system from the handlebars. This type of system also allows a simple U shaped fork to be used and incorporates a damper in the telescoping mechanism located in the steerer tube. In terms of weight, this design eliminates the multiple fork tubes, incorporates less oil and thus is inherently lighter weight than the previously discussed suspension systems.
In terms of weight savings, this design is still not ideal. The telescoping system described incorporates 6 to 8 steel races and 3 to 4 sets of bearings. Secondly, the highly stressed inner tube must be formed of a material and in a manner such that it bends rather than breaks. High strength steel is commonly used for the inner tube for this purpose, but it is heavy and counter to the consumer's preference. Lighter materials such as Aluminum in combination with strengthening processes such as shot-peening to strengthen the outer skin have been used as a material for the inner tube. Here-to-for, unfortunately, telescope assemblies whose inner tube connects to the fork crown with greater than 70 mm length of travel have been unable to pass stress testing using the present art as described in the aforementioned patents despite the additional costly process of shot-peening and use of expensive high-strength aluminum alloys. Kinzler et al. outlines methods to achieve a longer travel design through unique geometry and larger tube sizes. Regardless, this telescoping set of tubes weighs 450 g even using an inner tube of aluminum. One of steel would be even heavier.
Another prior art suspension system describes embodiments that can be characterized as a single-sided suspension system (U.S. Pat. No. 6,145,862). This type of system eliminates the weight of one side of the fork and allow for longer travel of the suspension. However, the system described uses 2 sets of clamps, a steerer tube piece to connect the clamps to the frame of the bicycle and a telescoping system coupled to a bent tube that clamps to the wheel of the bicycle. This design is heavier than required because of the clamps, additional tubing required in the steerer tube, the extended telescoping assembly and the bent tubing assembly attached to the wheel. In addition, this design utilizes the standard 3 or 4 flat system that incorporates pre-loaded bearings to reduce stiction; each set of flats utilized adds weight.
To achieve a light weight system, it would be ideal to incorporate the benefits of a needle bearing system with a design that utilizes a one-leg system. The design should have low stiction, but allowed good rotational control with the appropriate stiffness needed for a suspension fork.